Composite overwrapped pressure vessels (COPVs) are composed of a fiber-based composite overwrapping a polymeric or metallic liner. The liner serves as a permeability barrier, and the composite provides structural integrity and strength. The vessels are used as pressurization systems and to contain a variety of fluids on the space shuttle orbiter and on the International Space Station. Main propulsion system vessels supply helium for manifold repressurization, purging, pneumatic valve actuation, emergency shutdown, and propellant dumps. Reaction control system (RCS) tanks store and provide helium for the pressurization of RCS thrusters, and orbital maneuvering system (OMS) vessels provide propellant to the OMS engines.
An effort led by NASA White Sands Test Facility (WSTF) is underway to characterize spare fleet leader COPVs, including burst tests to compare residual and design strengths and to assess the possibility of the creep deformation of Kevlar (DuPont) fibers. The NASA Nondestructive Evaluation Working Group is fabricating new Kevlar bottles for accelerated aging tests. The NASA Glenn Research Center supported both of these efforts by performing fractography of samples from ruptured fleet leader vessels, as well as from stress-ruptured Kevlar strand tests at Texas Research Institute. Techniques for polishing Kevlar fibers and composites were developed, enabling Glenn to characterize polished cross sections of composites and mounted strands. As bottles from the accelerated aging program become available, they will undergo microstructural characterization.
Microstructural analysis (optical and field emission scanning electron microscopy is being used at Glenn to support the development of nondestructive evaluation methods, as well as additional Glenn efforts focused on characterizing constitutive properties and residual stresses.
Polished cross section from fleet leader tank serial number (S/N) 032 showing close packing of fibers and fiber deformation.
The micrographs from ruptured fleet leader COPVs illustrate representative findings. Deformation of fibers in transverse compression resulted in regions in which the fiber cross section began to deviate from round to "square" (see the preceding figure), with little matrix seen between adjacent fibers. Some cross-sectional areas show damage around fiber edges, which may denote actual fiber damage or weaker fibers, which are more readily damaged in polishing (see the next figure).
Polished cross section from fleet leader tank S/N 007 showing fiber damage at edges.
Fractography of ruptured fiber bundles illustrates the hierarchical structure of Kevlar fibers, which is composed of bundles of fibrils that split apart upon fracture (see the following figures).
Fractography of ruptured fiber from fleet lead tank S/N 011 showing branchlike fracture arising from hierarchical structure of Kevlar fiber.
Fractography of ruptured fiber from fleet lead tank S/N 011 showing “string-cheese” failure.
Fractography of ruptured fiber from fleet lead tank S/N 011 showing parallel sheet structure within Kevlar fiber.
Last updated: December 15, 2007
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